With rapidly increasing development of IOT (internet of things), it is desirable to provide a gas sensor which is capable of being combined with a portable/wearable device. In order to be integrable with the portable/wearable device, it is important to develop a gas sensor which is miniaturized, which is of low power consumption, and which is module-packaged. Generally speaking, miniaturization of the gas sensor may be achieved via a standard CMOS-MEMS (complementary metal-oxide-silicon and microelectromechanical system) process.
Referring to FIG. 1, a conventional gas sensor includes a substrate 10, an electronic circuit unit 11, and a heating-and-sensing unit 12. The electronic circuit unit 11 is primarily constituted of a drive circuit, a signal-reading circuit, and a temperature-controlling circuit, and is used for controlling the temperature of the heating-and-sensing unit 12 and for reading the signal from the heating-and-sensing unit 12. The heating-and-sensing unit 12 includes a micro-heater 121 disposed above the substrate 10, a plurality of sensing electrodes 122 disposed above the micro-heater 121, and a gas-sensitive layer 123 covering the sensing electrodes 122. The substrate 10 is formed with a cavity 100 located below the micro-heater 121 for dissipating the heat produced by the micro-heater 121. The configurations of the micro-heater 121, the sensing electrodes 122, and the gas-sensitive layer 123 may be designed according to the material characteristics therefor via the standard CMOS-MEMS process.
Polycrystalline silicon is usually used for making the micro-heater 121 so as to withstand a high temperature at which the micro-heater 121 is operated. The sensing electrodes 122 are made from gold or platinum. However, since the sensing electrodes 122 are made in an additional step performed after the CMOS-MEMS process, the overall process for manufacturing the conventional gas sensor is relatively complicated. In addition, since the material (i.e., gold or platinum) for forming the sensing electrodes 122 is expensive, the cost for manufacturing the conventional gas sensor is relatively high.
U.S. Pat. No. 7,495,300 B2 discloses a gas-sensing semiconductor device having a configuration similar to that of the conventional gas sensor shown in FIG. 1. As described in the abstract of U.S. Pat. No. 7,495,300 B2, the gas-sensing semiconductor device is fabricated on a silicon substrate and includes at least one sensing area provided with a gas-sensing layer separated from a heater by an insulating layer. As one of the final fabrication steps, the substrate is back-etched so as to form a thin membrane in the sensing area. The back-etch and the gas-sensing layer formation are carried out post-CMOS.
Referring to FIG. 2, another conventional gas sensor is illustrated in which the sensing electrodes 122 of the heating-and-sensing unit 12 may be made from the polycrystalline silicon via the standard CMOS-MEMS process. However, the sensing electrodes 122 should be arranged at two opposite sides of the micro-heater 121. As a result, the gas sensor manufactured thereby has a relatively large size, and the heat radiated by the micro-heater 121 may not be efficiently conducted to the sensing electrodes 122.
Therefore, it is desirable in the art to develop a gas sensor which may overcome the aforesaid disadvantages of the conventional gas sensors.